1. Field of the Invention
The invention relates to an amplifier, and more particularly, to an amplifier capable of compensating offset voltage thereof.
2. Description of the Related Art
Conventional amplifiers comprise two input terminals and one output terminal. The amplifier is supplied with direct current (DC) from two DC power sources. In operation, the amplifier measures the difference between voltages of two input terminals and multiples the difference with a gain to generate a voltage in the output terminal. Ideally, the input terminals are assumed to have very high impedance so minimal current flow into or out of the input terminals. Therefore, the signal current of the two input terminals are assumed to be zero, such that output impedance approaches an infinite level.
Consequently, an operational amplifier only responds to a signal difference between voltages of the two input terminals with no consideration of the common part therebetween. That is, the output is assumed to be zero when the two input voltages are the same, referred to as common mode rejection. Gain between the two input terminals is referred to as differential gain, which retains a constant value in a frequency range from zero to infinity. Ideally, an ideal operational amplifier amplifies any frequency with the same gain value. However, the offset voltage of the operational amplifier will not be zero, being influenced by process drift, element matching, or others.
In a digital to analog converter, once the operational amplifier has an offset voltage, the signal is distorted, and unexpected effects can be applied to the circuit coupled behind the operational amplifier. Particularly, the operational amplifier cannot have an offset voltage when measuring a small signal, since the voltage of the signal source itself is small. Therefore, it is important to eliminate the offset voltage for the operational amplifier without increasing circuit area and power consumption.